This invention relates to a pulse generator for generating pulse groups each comprising a predetermined number of pulses of given pulse widths and given relations within the group, said pulses being used for example as synchronization and triggering pulses in a radar system, and in which the pulse generation is carried out purely digtally using digital circuit elements and depending on a control frequency, which is generated by a crystal oscillator provided therein or, alternatively, which is supplied externally.
A pulse generator embodying the invention is useful in different applications in which trigger and synchronization pulses generated with a high level of frequency stability are required. One specific application for such a pulse generator is as a synchronization generator in a MTI radar system of the pulse Doppler type, that is a radar system using the Doppler effect for obtaining velocity information with respect to targets.
The echo signals of a moving target as well as the received clutter vary in phase and amplitude. A pulse Doppler radar utilizes the phase variation of the echo signal to recognize the Doppler component produced by a moving target. A pulse Doppler radar, therefore, must be either transmitter coherent or receiver coherent. This makes it necessary to use a frequency stable synchronization generator which directly or indirectly will take care of the time control of the transmitter and receiver elements.
Dependent on the area of application of the pulse doppler radar system different radar pulse repetition frequencies (PRF's) are desirable, which in turn demands that the pulse groups required for the function of the radar and supplied by the synchronizing generator are repeated at a selected PRF, which may bring with it changed pulse widths and pulse distances within the group.
One problem with pulse doppler radars is that the of so-called blind velocities are directly proportional to the PRF. Varying the PRF may enable a specific targer to be visible at at least one frequency. The variation may be obtained by so-called staggering, that is alternatively short and long radar pulse intervals.
In order to prevent the radar system from being jammed by interference so called jittering of the PRF may be used, that is varying the frequency randomly within a frequency interval around a selected PRF.
An object of the invention is to provide a digital pulse generator which is suitable for use as a synchronizing pulse generator in a radar system and which makes possible a selection of PRF and a desired mode of operation, for example staggering or jittering, only by the supply of fixed PRF selection codes and mode selection codes, and in which a very high frequency stability is maintained independent of the functional mode and thereby making possible a good measurement accuracy and clutter suppression.